Norbornyl cation hydride shifts

The tabulation shows that the t-butyl system is a reasonable model for some equilibrating ions. It fails badly, however, when applied to the norbornyl compounds. The isopropyl system is a poor model for sec-butyl and cyclopentyl ions and is a very poor model for the norbornyl cation. The failure of the models to provide reasonable estimates of the shifts in the tertiary norbornyl cations which are undergoing either VVagner-Meerwein shifts or hydride migration makes it clear that the experimentad shifts in the secondary system cannot be used as structural proofs. Rather they should be regarded as fascinating results to be rationadized in terms of the structure, whatever it may be. 

The proton nmr behaviour of the norbornyl ion provides a wealth of information which, however, dso appears to be of limited value in the structural problem. The cation has been observed by H-nmr in many solutions containing SbFs as well as in GaBr3—SO2 Qensen and Beck, 1966). At —80° in SbFs—SO2 it exhibits 3 peaks, 5T86 (area 6), 2 82 (area 1) and 5 01 (area 4). The assignments and experimental coupling constants are shown in Figure 5 (Olah et al., 1970). The 5-01 p.p.m. peak indicates equivalence of the four protons, which is caused by rapid 6,1,2-hydride shifts. 

Of particular importance are recent results on the energetics of the exo-2,3-hydride shift in the 2,3-dimethyl-2-norbornyl and the l,2,3,4-tetramethyl-2-norbornyl cations. Line broadening studies reveal a free energy of activation of 6-6 and 7-3 kcal mole for this process in the respective ions (Huang eta/., 1973 Jones et a/., 1974). 

In summary, the temperature dependence of the proton nmr spectra does not provide a sufficient basis to decide upon the norbornyl cation structure. The slow 3,2-hydride shift might be rationalized by either a non-classical or classical interpretation, but both the activation energies and the pre-exponential factors might be influenced by solvation. If this is the case the arguments based on the pmr observations become still less persuasive. 

The complete Fourier transform C-nmr spectra (with all coupling constants and multiplicities) were obtained for the 2-norbornyl cation at —70°C and -150°C by Olah et al. (1973a). At -70°C in SbFs-SO ClF-SOjF the ion undergoes rapid equilibration via 6,2-hydride shifts and only one resonance was observed for cyclopropane-like carbons 101.8 (ref CSa, quintet, /isch = 53.3 Hz). This peak was split into two at —150°C. One peak... 

Some of these rearrangements have also been observed in the parent norbornyl cation [4] and the Jco-3,2-hydride shift (A(7 =11.4 kcal mol" ) and the e/idb-6,2-hydride shift AG = 5.8 kcal moh ) have been studied by dynamic H-nmr spectroscopy (Olah et al., 1970c). 

The 2-brexyl cation (2-tricyclo[4.3.0.0 - ]nonyl cation, [215]) has an intriguing structure containing two norbornyl moieties. Of the two obvious rearrangement paths possible for ion [215], Wagnei-Meerwein shifts (139) and 1,3-hydride shifts (140), only the former is degenerate (Nickon et ai, 1965). The latter gives the isomeric 4-brexyl cation [216], which can undergo further... 

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbornyl cation has also been extensively smdied at low temperatures there is much evidence that under these conditions the ion is definitely nonclassical. Olah and co-workers have prepared the 2-norbomyl cation in stable solutions at temperamres below 150°C in SbFs—SO2 and FSO3H SbF5 S02, where the stmcmre is static and hydride shifts are absent Studies by proton and NMR, as well as by laser Raman spectra and X-ray electron spectroscopy, led to the conclusion that under these conditions the ion is nonclassical. A similar result has been reported for the 2-norbomyl cation in the sohd state where at 77 and even 5 K, NMR spectra gave no evidence of the freezing out of a single classical ion. ... 

Rearrangements of carbocations also may be. studied by NMR methods. The norbornyl cation (5-26) may undergo 3,2- and 6,2-hydride shifts, as well as Wagner-Meerwein (W-M)... 

Since in electron spectroscopy the time scale of the measured ionization processes is on the order of 10 16 sec, definite ionic species are characterized, regardless on their possible intra- and intermolecular interactions (e. g., Wagner-Meerwein rearrangements, hydride shifts, proton exchange, etc.). Thus, electron spectroscopy gives an undisputible, direct answer to the long debated question of the non-classical nature of the norbornyl cation independent of any possible equilibration process. 

The excess of 1 over 2 indicates that some syn addition occurs by ion pair collapse before the bridged ion achieves symmetry with respect to the chloride ion. If the amount of 2 is taken as an indication of the extent of bridged ion involvement, one can conclude that 82% of the reaction proceeds through this intermediate, which must give equal amounts of 1 and 2. Product 3 results from the C(6) C(2) hydride shift that is known to occur in the 2-norbornyl cation with an activation energy of about 6 kcal/mol (see p. 450). 

Within —50 to —130 °C there are 3 PMRsignals with an intensity ratio of 4 1 6. This points to the freezing of 3,2-hydride shifts (E = 10.8 0.6 kcal/mole A = 10 - s ). Judging from these data the 3,2-hydride shift rate in a stable 2-norbomyl cation is abnormally low compared with 1,2-hydride shifts in secondary carbocations. Thus the respective activation energies are 5 kcal/mole for the 1,2-hydride shift in the cyclopentyl cation and 10.8 kcal/mole for the 3,2-hydride shift for the 2-norbornyl cation. This corresponds to the rate ratio 10 at —150 °C and 10 at 25 °C. Olah has studied the models of both ions showing that torsional and non-... 

In 2,3-dimethyl-2-norbornyl cations the rate of the exo-3,2-hydride shift is far higher than that of the endo-3,2-hydride shift (the difference in activation energies 5.5 kcal/mole Contrary to Olah s data of the two isomeric ions-2,3-endo-dimethyl-2-norbornyl 13S and 2,3-exo-dimethyl-2-norbornyl 139 the more stable one is the former As has been noted earlier, the vacant p-orbital at C interacts mostly with the exo substituent at C this hyperconjugative interaction with the C —H bond is more effective than with the C —CHj bond. 

Berson, j. a., j. H. Hammons, A. W. McRowe, R. G. Bergman, A. Remanick, and D. Houston The Chemistry of Mcthylnorbomyl Cations. VI. The Stereochemistry of Vicinal Hydride Shift. Evidence for the Nonclassical Structure of 3-Methyl-2-norbornyl Cations. J. Amer. Chem. Soc. 89, 2590 (1967). 

Although 1,3-hydride shifts are occasionally observed, they do not compete favorably with 1,2-shifts in alicyclic systems. In certain bicyclic systems, they become more favorable. In the the norbornyl cation, for example, the 6,2-shift of hydrogen is more favorable than the 2,3-shift ... 

One cannot fix isotopic labels in the 2-norbornyl cation because at temperatures above — 130°C any hydrogen or deuterium is equilibrated among all 11 positions in a few minutes due to the slow (at these temperatures) 3,2-hydride shift. The deuteriated cation will be a mixture of the two types of labelled cations. Neglecting an isotope effect on the 2,3-hydride shift, 7/11 of the cations will have one deuterium located at C-3,5,7 or C-4. In these cations the 6,2,1-shift is not perturbed and a peak at the normal unperturbed position is observed. The remainder (4/11) of the cations will have one deuterium located at one or other of the C-1,2,6 positions. 

The hydride shifts of norbornyl cation are well accepted it is the carbon shift that has generated so much controversy. The issue is whether Eq. 11.38 is very facile, or whether we have a single symmetrical structure (see the drawing below). A large fraction of physical or-... 

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